1. Field of the Invention
This invention relates to charged capacitive transducers, and in particular to diaphragm configurations for transducers which are in contact with stator elements of an electrostatic speaker.
2. Prior Art
Although the concept of electrostatic loudspeakers has been in development for about one hundred years, commercial applications were not realized until the late 1940's. Since then, a standard construction for electrostatic loudspeakers has dominated the state of the art. This standard utilizes two conductive, stationary, planar stators positioned such that they are in parallel, coaxial relationship with a specific displacement gap therebetween. A tensioned diaphragm, usually made of metalized polyester, is positioned between the stators in noncontacting, suspended configuration. Two polarities of an AC audio signal are sent from the output of a step-up tansformer to the two respective stators. A high voltage, low current DC bias voltage is concurrently applied to the movable diaphragm for enabling vibration for an audio output.
Despite many advantages of electrostatic speakers, commercial acceptance over dynamic speaker systems has been nominal. Magnetically driven cones and related dynamic speakers have secured over 90% of the audio market, and continue to increase in market share. This is due in part to the high cost of production of quality electrostatic systems, space requirements for obtaining low range frequency response, and difficulty in obtaining structures which are suited for long term stability and dependable operation.
Technical difficulties with prior art electrostatic loudspeakers include balance of diaphragm tension, resonant frequency, bias voltage and diaphragm stability. Such balancing is problematic, because adjustment of one factor often leads to imbalance of another. For example, greater tension is required for stability; however, any increase in tension results in concurrent reduction of low frequency range response. Similarly, lowering the bias voltage increases stability, but at the same time reduces sensitivity. Besides trade-offs in performance, the tensioning of the diaphragm leads to manufacturing difficulties and challenges with maintenance of required tension over extended periods of use. Tensioning requirements of suspending the diaphragm in noncontacting condition between the stators severely limits speaker shape and configuration. Accordingly, prior art electrostatic speakers have traditionally been expense, flat, large in size and generally unattractive.
Another issue with prior art electrostatics is that they are very sensitive to loading and reflections that come from any wall or enclosure which is intimate with the speaker device. The result can be an undesirable increase in resonant frequency, reduced midband output, and poor frequency response. These factors place significant limitations on speaker positioning within a home or commercial application, and raise further concerns with customer satisfaction with the field of electrostatic systems generally.
U.S. Pat. No. 2,872,532 by Buchmann et. al.; U.S. Pat. No. 2,935,575 by Bobb; and U.S. Pat. No. 4,439,642 by Reynard are representative of numerous prior art references which teach the basics of electrostatic speaker design. These references illustrate the conventional tensioned diaphragm suspended over a stator which includes openings or some other form of adaptation for acoustical transparency. Numerous variations of design have been attempted to realize other uses for electrostatic systems. For example, U.S. patent by Schindel et. al. shows modification of the rigid stator to include a roughened surface with peaks and valleys (FIG. 4) to facilitate high frequency response useful for ultrasonic emitters. U.S. Pat. No. 2,855,467 by Curry and U.S. Pat. No. 3,544,733 by Reylek et. al. illustrate flexible dielectric diaphragms in combination with flexible conductive films which provide marginal audio output for specialized applications. Other advances have been pursued by anchoring or attaching the diaphragm along its surface area to avoid the difficulty of broad surface tensioning as discussed above. FIG. 6 of U.S. Pat. No. 1,809,754 by Steedle shows the capture of a diaphragm 16 between opposing stator elements 10 and 11. This construction localizes the vibrating diaphragm area to strips as illustrated in FIG. 3 and develops a nonplanar diaphragm configuration. A similar "strip-like" construction is illustrated in U.S. Pat. No. 1,799,053 by Mache wherein a rigid stator "a" supports a nonplanar diaphragm "b". Hereagain, diaphragm movement appears to be isolated to strips of material captured as shown in FIG. 3.
None of the prior art references have developed a commercially acceptable solution to the difficulty of providing an inexpensive electrostatic speaker that can service the audio industry.
Nevertheless, such applications continue to become more extensive as quality sound systems are being demanded with virtually every sound source from pocket CD players and lap top computers, to surround sound stereo associated with in-home theater applications. Such systems require broad frequency response, maintenance free use, and cost competitive basis to enable competition with the dynamic speaker market.